How To Power Multiple 4-20 mA Sensors

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We’re often asked how to power multiple 4-20 mA sensors using a single power supply. While it is possible to do this, the usual cautions apply regarding ground loops and other subtleties. Readers who are new to measurements using sensors with 4-20 mA process current outputs should refer to our earlier article that explains basic concepts and configurations.

Multiple 4-20 mA Sensors, One Power Supply

So, you have multiple 2-wire sensors with 4-20 mA outputs and only one power supply. How can you make this work? Use this step-by-step process:

  1. Take an inventory of the minimum and maximum power supply requirements for each sensor. Choose the highest of the minimum and lowest of the maximum values of all sensors. Then pick a power supply voltage that is roughly half way between these two figures. For example, if the highest of the minimum power supply voltage for five sensors  is 10 V, and lowest of the maximum power supply voltage is 20 V, pick a power supply voltage of about 15 V. The actual value isn’t critical provided that you give yourself breathing room of a couple of volts or so from the two extremes.
  2. Multiply the number of 4-20 mA sensors to be powered by 0.02 and add 20%. This is the maximum current that the power supply is required to deliver in amperes. Again for five sensors, this value is 5 × 0.02 × 1.2 = .12 Adc, or 120 mA.
  3. Unless the instrument used to make the measurement has built-in shunts, you’ll need to add these externally. There are correct and incorrect placements depending upon the instrument, so refer to our earlier article here for complete information.
  4. Connect your sensors, power supply, and shunts (if required) as shown in Figure 1, where:
    • “E” represents the power supply
    • “R” is the shunt resistor. Replace “R” with an open circuit if the instrument has built-in shunt resistors.
    • “V” is the signal connected to the instrument.

That’s all there is to it. Using these guidelines there is no practical limit to the number of 4-20 mA sensors that can be powered by a single power supply, saving space and money.

Schematic of two or more 2-wire 4-20 mA sensors powered by a single power supply.

Figure 1 — Schematic of two or more 2-wire 4-20 mA sensors powered by a single power supply. (click to enlarge)

 

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